The invention relates to a side-lobe blanking circuit for a pulse radar apparatus, provided with a transmitting and receiving unit containing a transmitter, a directional transmitting and receiving antenna, a first receiver containing a pulse compression filter and an omnidirectional receiving antenna, and a second receiver containing a pulse compression filter. The transmitting and receiving unit is suitable for the transmission, reception and processing of relatively long-duration and relatively short-duration radar pulses into video signals in the two receivers. At least the relatively long-duration pulses are frequency-modulated. The radar apparatus is further provided with first switching means, whereby in response to the video signals from the two receivers the side-lobe blanking circuit supplies a first switching signal to block, when applied to the switching means, the transport of the video signals from the first receiver to the switching means.
The beam pattern of a directional transmitting and receiving antenna contains, in addition to its main lobe, several side lobes, which are usually 30 to 40 dB weaker than the main lobe. However, even in the case of an ideal beam pattern, apparent side lobes could arise because of obstacles in the vicinity of the antenna. For instance, in a ship radar system, strong targets, such as islands, offshore oil rigs, supertankers, etc. may give rise to radar echoes in the order of 80 to 120 dB above noise level. These echoes prevail not only in the main lobe, but also in the side lobes of the antenna beam pattern, give rise to annular echoes when presented on a PPI display, and require more complicated video processing and extraction. For this reason means have been developed to distinguish echoes received through side lobes from echoes received through the main lobe. These means include an omnidirectional antenna; this is an antenna that is sensitive in all directions and in elevation especially for the horizon, from which strong echoes can be expected. The omnidirectional antenna is required to possess a gain, which is greater than the gain of the directional antenna in the side lobes of the beam pattern thereof. Side-lobe blanking is based on the principle that, as soon as the echo strength through the receiver connected to the omnidirectional antenna is greater than that through the receiver connected to the directional antenna, the echo is evidently not from a target contained in the main lobe of the beam pattern and further transport of the video signal from this echo in the receiver connected to the directional antenna is interrupted. This principle has been known for a long time.
In a pulse radar apparatus, where frequency-modulated transmitter signals are emitted and where in the receiver of this apparatus the received and processed echoes are compressed accordingly through a suitable filter, side-lobe blanking can equally be applied. In such a case, the receiver connected to the omnidirectional antenna will also be provided with a pulse compression filter. Pulse compression improves the signal/noise ratio by about 20 dB; this means that certain annular echoes received through the side lobes and detected without compression might not be displayed on a PPI, but might be so displayed if they are compressed before detection. It is therefore possible to apply side-lobe blanking according to the aforementioned principle, viz. by a side-lobe blanking circuit that, in response to both the echoes compressed and detected in the receiver connected to the directional antenna and the echoes compressed and detected in the receiver connected to the omnidirectional antenna, interrupts any further transport of the echoes compressed and detected in the receiver connected to the directional antenna. However, the side-lobe blanking circuit does not operate satisfactorily because of the non-ideal operation of the pulse compression filters. Such a filter not only compresses the pulse, for instance a 50 .mu.s pulse to an approximately 0.5 .mu.s pulse, but also produces several time side lobes at a level of, say, -40 dB. For this reason the dynamics of the receivers' part preceding the pulse compression filter is limited, in the cited example to about 40 dB. This means that strong target echoes and hence annular echoes received through the side lobes may drive the receivers to saturation. In such a case it is not possible to make a choice between the received, compressed and detected echoes in the two receivers. Therefore, it cannot be determined whether the transport of the video signals from the first receiver has to be stopped; that is, it is not possible to distinguish between echoes received through the main lobe and the side lobes. This problem cannot be solved by feeding the non-compressed echoes to the side-lobe blanking circuit. The above argument that it is possible to display on a PPI the annular echoes received through the side lobes and compressed before detection may not be so for non-compressed and detected echoes. It should be noted, however, that the non-compressed pulse may be of a relatively long duration, for example 50 .mu.s, thus covering a large distance, so that blanking during this period affects the operation of the pulse radar apparatus.